1. Field of the Invention
This invention relates to photographic cameras, and more particularly to cameras provided with automatic sharp focus detecting devices for electrically detecting the sharpest focus position of a photo-taking lens relative to an object to be photographed or with automatic focus adjusting devices for automaticallyv adjusting a photo-taking lens to its sharpest focus position.
2. Description of the Prior Art
Many devices have been proposed for automatically detecting the sharpest focus position of a photo-taking lens relative to an object to be photographed and for automatically adjusting a photo-taking lens to the sharpest focus position. And, many efforts have been devoted to practical application of these devices to cameras of reduced size.
Regrettably, however, conventional types of automatic sharp focus detecting devices or automatic focus adjusting devices can not function perfectly under all the conditions, and are unavoidably limited in performance. Thus it is impossible to detect the condition of sharpest focus in some given photographic situations. For example, these devices may be classified into two main groups, namely, what is called "passive" type where ambient illumination on an object to be photographed is utilized, and what is called "active" type where a radiation (visible light, infrared light and the like) is projected artificially onto the object, and the reflected radiation is utilized. The former type of device, because of the utilization of the radiation emanating from the object itself only, make it impossible to focus sharply when the object brightness is less than the radiation detectable level of the radiation sensor. On the other hand, in the latter type device, because of the independence of the radiation from the object itself, the disadvantage of the former type can be overcome. However, when the object has rough surfaces, or when the object is comparatively far, the intensity of radiation reflected is extremely low so that the detection of sharpest focus can not be performed properly. In addition thereto, this active device necessitates a radiation source such as a light projector which is generally of large consumption of electrical energy. On the other hand, electrical power source capacity in a camera of reduced size is limited, and therefore, the amount of electrical energy supplied for production of the radiation is limited. This also leads to the limitation of the possible maximum level of radiation energy to be projected. As a result, the range of object distances for which the sharp focus detection is possible becomes narrower and thereby increases the opportunity for focusing errors.
Another problem arising in practical application of the automatic sharp focus detecting devices or the automatic focus adjusting devices into the cameras of reduced size, is that, the capacity of the electrical energy source in the camera is limited. In general, these devices consume comparatively large amounts of electrical energy, regardless of whether the device is of the passive or active type, and can not operate properly when the supplied voltage drops below a satisfactory operating level for the device. It is of importance, therefore, to take this fact into account in the practical application of these devices into cameras of reduced size.
Many attempts have been made to overcome the above mentioned drawbacks, but have failed to provide decisive solutions therefor.
It is to be understood from the foregoing that, in practical application of these automatic focus detecting or focus adjusting devices into the cameras, it is advantageous to provide a supplementary device in a relieving sense for the purpose of making it possible to obtain a condition of approximate focusing, although not the perfectly sharpest focus, so that the photographer is enabled to take photographs under the permissible focusing condition when the detection of a condition of sharpest focus can not be performed in a satisfactory manner due to the above mentioned reasons.
In addition to this, from the standpoint of the particular characteristics of photographic cameras, it is further advantageous to make an exposure control apparatus cooperative with the focusing system in such a manner that, when the focus detection can not be performed satisfactorily, the exposure control apparatus is rendered inoperative, and only when the photo-taking lens is properly focused on the object by the use of the automatic sharp focus detecting device or the automatic focus adjusting device, or when the taking lens is adjusted to the approximated focus position by the use of the supplementary device, said exposure control apparatus is rendered operative automatically, thereby the otherwise wasteful consumption of film due to the making of exposures with an unacceptable lack of sharpness of images can be prevented.